The present invention relates to pressure compensation in hydraulic vehicle suspension systems.
In vehicle suspension systems, it is often desirable to provide a relatively high roll stiffness so that the attitude of a vehicle equipped with the system remains relatively stable during cornering. It is also often desirable to provide a relatively low cross-axle articulation stiffness so that as the vehicle traverses uneven terrain wheels of the vehicle are permitted to articulate and thereby substantially maintain equal loading on the wheels. These characteristics are especially important for vehicles which have high roll moments, including most all-wheel drive off-road vehicles and all trucks.
A passive suspension system is disclosed in International Patent Application WO 97/06971. Referring specifically to FIGS. 5 to 13 which show suspension systems utilising hydraulic interconnections, a left front wheel of a vehicle is operatively associated with a piston of a front double acting hydraulic ram, a right front wheel of the vehicle is operatively associated with a cylinder of the front hydraulic ram, a left rear wheel of the vehicle is operatively associated with a piston of a rear double acting hydraulic ram, and a right rear wheel is operatively associated with a cylinder of the rear hydraulic ram. Chambers of the front hydraulic ram are connected to corresponding chambers of the rear hydraulic ram by fluid links so as to form a hydraulic circuit comprising two enclosed fluid volumes. The arrangement is such that movement of leftmost wheels of the vehicle in an opposite direction relative to the body of the vehicle than rightmost wheels of the vehicle tends to effect compression of fluid in one of the fluid volumes. This has the effect of restraining movement of the wheels relative to the vehicle body and of resisting roll of the vehicle during cornering. The arrangement is also such that movement of a first set of diagonally opposite wheels in an opposite direction relative to the body of the vehicle than a second set of diagonally opposite wheels tends to circulate fluid in the hydraulic circuit. This has the effect of permitting movement of the wheels relative to the vehicle body, thereby permitting relative motion of transversely adjacent wheels in a cross-axle articulation (or warp) motion.
Although the suspension system operates satisfactorily in passively reducing roll of the vehicle whilst permitting cross-axle articulation, as the temperature adjacent the suspension system varies, the temperature and therefore the volume of fluid in the two fluid volumes also varies. As the expansion of the fluid with temperature increases the volume of fluid within each fluid volume, said fluid volumes must expand to accommodate the excess fluid. There is a usually a limited amount of expansion available due to component resilience, however a high component stiffness is desired for the roll control operation of the system. This high stiffness can cause a significant increase in pressure in the system with temperature. If the pressure becomes too high, friction in seals of the suspension system will become significant and, ultimately, may lead to significant egress of fluid and failure of the suspension system. If the pressure becomes too low, air remaining in the hydraulic circuits becomes significant and the ability of the suspension system to generate a high roll stiffness is reduced.
In addition, during normal operation of the suspension system small amounts of fluid are continually lost through the seals between the piston and the cylinder of each hydraulic ram, this fluid loss also contributing to a reduced system pressure and a consequent reduction in roll stiffness.
Temperature differences and egress of fluid may also lead to the static effect cylinder force changing which can cause differences in left and right handling of the vehicle.
The present invention seeks, among other things, to provide a hydraulic system for a vehicle suspension which overcomes at least one of the above mentioned disadvantages. In accordance with one aspect of the present invention, there is provided a hydraulic system for a vehicle suspension, said hydraulic system being a portion of a roll control component of said vehicle suspension, said hydraulic system including
a front hydraulic ram means and a rear hydraulic ram means, the front and rear hydraulic ram means being respectively in the form of a double acting hydraulic ram and including a cylinder portion and piston portion, the piston portion separating the cylinder portion into first and second chambers;
a first fluid line extending, in use, between the first chamber of the front hydraulic ram means and the first chamber of the rear hydraulic ram means, thereby forming a first fluid volume,
a second fluid line extending, in use, between the second chamber of the front hydraulic ram means and the second chamber of the rear hydraulic ram means, thereby forming a second fluid volume,
at least one accumulator means in fluid communication with at least one said fluid volume, the accumulator means including a sealed gas enclosure and a fluid enclosure in fluid communication with said at least one fluid volume; and
fluid flow restricting means adapted to restrict the rate of fluid flow between the fluid enclosure and the corresponding at least one fluid volume such that there is little to negligible change in the volume of the gas enclosure during cornering of the vehicle, the volume of the gas enclosure varying over time for at least substantially equalising the pressure in the fluid volume and the gas enclosure thereby minimising variations in the pressure within the fluid volume.
The hydraulic system may form part of a suspension system as described in International Patent Application WO 97/06971 referred to previously. It is however to be appreciated that the hydraulic system can also be used on other hydraulic vehicle suspension systems utilising interconnected hydraulic ram means.
The hydraulic rams may be of the through rod type where the first and second chambers are respectively provided with the same piston face area such that the pressures in the fluid volumes are generally symmetrical. It is however also possible to use more conventional hydraulic rams having unequal piston areas.
According to one preferred embodiment of the present invention, the accumulator means may be in fluid communication with both fluid volumes. The accumulator means may include an expansion accumulator, and the flow restricting means may be in the form of first and second capillary tubes respectively interconnecting the expansion accumulator with the first and second fluid conduits. Other forms of fluid flow restricting means are however also envisaged.
Preferably, the fluid flow restriction means is adapted to provide enough flow to equalise the pressure difference between the first and second fluid volumes caused by static differences such as temperature differences and leakage, to allow the vehicle to return to a substantially level position from an offset position. The fluid flow restriction means should also be restrictive enough to limit the flow of fluid caused by dynamic pressure differences, such as those generated by the vehicle cornering.
The expansion accumulator may include a cavity separated by a flexible membrane into a gas enclosure and fluid enclosure, the fluid enclosure being in fluid communication with the first and second fluid volumes.
According to another preferred embodiment, the hydraulic system may include a separate accumulator means for each hydraulic volume, the flow restricting means being in the form of a capillary tube interconnecting each accumulator means to a respective said hydraulic volume.
The hydraulic system according to the present invention may further include a valve means located between the or each accumulator means and the hydraulic volumes. In the first described embodiment, the valve means may be provided between the accumulator and the flow restricting means. It is also envisaged that a said valve means be provided between each accumulator and hydraulic volume in the second described embodiment.
In one form, the valve means may be in series with the flow restricting means. In this form, the valve means can be small and is relatively safe in operation, as fluid flow must pass through the fluid flow restricting means before passing through the valve means.
In another form, the valve means may be in parallel to the fluid restricting means. The valve means would preferably be open to allow some pressure equalisation between the volumes and may be closed at other times, including when the vehicle is cornering. The valve means may be subject to high fluid flow, which can lead to undesirable movements in the suspension. Therefore, additional mechanisms may need to be employed to reduce the flow of fluid across the valve means upon opening, such as partially opening the valve means, or including additional flow dampening means.
The valve means may include a solenoid and a valve member reciprocally movable between a closed valve position and an open valve position, the valve member being biased towards the closed position by a resilient means, the actuation of the solenoid effecting movement of the valve member towards the open valve position. The valve means may open to thereby provide fluid communication between the two hydraulic volumes and the accumulator means. The valve means may open periodically or may open as a function of vehicle motion. Sensor means may be provided to sense, for example, cornering of the vehicle such that the valve means is closed during cornering. This provides for low roll stiffness during straight line motion, and a high roll stiffness during roll motion.
The hydraulic system according to the present invention is therefore adapted to take into account changes in fluid volume and pressure due to fluid leakage, change in temperature and so on.
According to another aspect of the present invention, there is provided a roll control component for a vehicle suspension including a hydraulic system according to the present invention.
The roll control component may include a front and rear roll stabiliser bar, each stabiliser bar respectively interconnecting wheel assemblies of the vehicle. The front and rear hydraulic ram means may respectively interconnect the front and rear stabiliser bars to the body of the vehicle. At least the rear stabiliser bar may be pivotally mounted to the vehicle. The front stabiliser bar may be mounted by a drop link to the vehicle body.